1. Field of the Invention
The invention is related to a tool to slice ingots (i.e. boules or bulk crystals) of group III nitride crystals such as gallium nitride (GaN), indium nitride (InN), aluminum nitride (AlN) and their solid solutions. Also, the invention discloses a method to slice ingots of group III nitride crystals to fabricate wafers of group III crystals.
2. Description of the Existing Technology and Some of its Problems
Slicing a crystal ingot is the most common way to obtain crystalline wafers used in various applications such as electronic or optoelectronic devices. A multiple wire saw, an outer blade saw, and an inner blade saw are commonly used tools to slice wafers out of a crystal ingot. Among these methods, the multiple wire saw is more commonly used than the blade saws because the multiple wire saw enables multiple slices in one run and the slicing margin is thinner than that of solid blades. The slicing margin of the multiple wire saw is determined by the wire thickness and the slurry size. It is typically between 150 to 200 microns, depending on the hardness of the crystal and slicing conditions. The multiple wire saw is widely used to slice semiconductor crystals such as silicon (Si), gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP), and silicon carbide (SiC).
Gallium nitride (GaN) and its related solid solutions explained as AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1) are highly enabling materials in optoelectronic and electronic devices. Since GaN and its related solid solutions do not have a liquid phase under the standard pressure, it has been extremely difficult to grow the crystal in ingot shape. Recent progress in crystal growth technology enabled ingot growth of III-nitrides; however, a decent slicing technique has not fully been developed yet. The (U.S. Pat. No. 7,464,702 B2)[1] discloses one method of slicing GaN ingots with a multiple wire saw. GaN wafers are sliced from GaN ingots with a slicing margin of 160 to 280 microns. Since the typical wafer thickness is 300˜400 microns, the kerf loss (i.e. the loss for the slicing margin) can be as high as 48%. Considering the cost of growing III-nitride ingots, there is a strong demand to minimize the slicing margin. However, it is quite difficult for the multiple wire saw to attain slicing margin smaller than 100 microns because thinner wires will not have sufficient strength to slice crystal ingots, especially hard materials such as GaN.